Construction began during the Warring States Period (403-221 B.C.), when separate sections were built in scattered strategic areas. Following China's unification under the first Qin emperor, Qin Shi Huangdi (221-206 B.C.), 300,000 men (mainly political prisoners) were put to work connecting the segments into one continuous rampart (at one time stretching 10,000 km, 6200 mi.). The following set covers the 'Ba Da Ling' section, 75 km northwest of Beijing, which dates to the Ming Dynasty (1368-1644) and was planned by General Qi Ji Guang.